As absorbent resins, cross-linked partially neutralized polyacrylic acid, saponified acrylic ester-vinyl acetate copolymers, modified cross-linked polyvinyl alcohols, cross-linked isobutylene-maleic anhydride copolymer, hydrolyzed starch-acrylonitrile graft polymer, starch-acrylic acid graft polymer, and partially crosslinked polyethylene oxide have been known to the art. These absorbent resins are widely used as sanitary napkins, disposable diapers, and other similar sanitary absorbent agents and as water-retaining agents and dehydrators in the field of agriculture and horticulture and the field of civil engineering, for example.
The absorbent resins are generally used in the form of fine powder or minute particles. In many cases of actual use, such an absorbent resin is converted preparatorily into a composite in the form of a sheet or a film as by being sandwiched with opposed sheets of paper, incorporated in pulp and compressed together by the use of the embossing technique, or melt sealed as with a thermoplastic resin.
Various proposals have been heretofore made for the purpose of realizing the use of the absorbent resin in the form of a sheet by a general procedure of depositing the absorbent resin on a substrate and subjecting the resultant composite to a treatment of fixation. They can be broadly divided into methods which comprise a first step of producing an absorbent resin and a subsequent step of molding the absorbent resin in the form of a sheet and methods which comprise a single step of effecting simultaneously the production of an absorbent resin and the molding of this resin in the form of a sheet. The former methods consist of (a) methods which comprise depositing an absorbent resin powder on a sheetlike substrate and subjecting the resultant composite to a treatment of fixation and (b) methods which comprise molding absorbent resin fibers in the form of a sheet. The methods of (b) are not very popular because the production of absorbent resin fibers itself is unduly expensive and the produced absorbent sheet is deficient in the ability to absorb water.
The methods (a) which are varied by the kind of treatment of fixation embrace a method which comprises spraying an absorbent resin powder on a substrate sheet, superposing another sheet thereon, and subjecting the resultant laminate to an embossing work, a method which comprises mixing an absorbent resin powder with a substrate sheet and subjecting the resultant mixture to an embossing work, methods which comprise imparting a small moisture content to an absorbent resin powder and ensuring powerful fixation of the absorbent resin power to a substrate sheet (such as are disclosed in U.S. Pat. No. 3,959,569, JP-A-51-40,497, JP-A-54-123,293, JP-A-54-141,099, and JP-A-58-36,452), and methods which comprise realizing fixation of an absorbent resin powder to a substrate sheet by the use of a resinous binder (such as are disclosed in JP-A-58-101,047, JP-A-4-504,234, and U.S. Pat. No. 5,128,082), for example. JP-A-1-230,671 proposes a method for immobilizing an absorbent resin by combining the absorbent resin with an aqueous liquid thereby forming a hydrate. These methods, however, are at a disadvantage in entailing certain difficulties in the production of an absorbent sheet. For example, they essentially require to use a substrate for the purpose of retaining the absorbent resin powder in the form of a sheet, entail a complicate process for the formation of a composite of an absorbent resin powder with a substrate, jeopardize the work environment because an absorbent resin powder is liable to drift, and produce a sheet deficient in strength or flexibility. Further, since they are not allowed to increase the amount of an absorbent resin powder to be deposited per unit area of the substrate, they suffer from a small capacity for absorption and a small speed of absorption. If the amount of the absorbent resin to be deposited per unit area of the substrate is increased, the produced sheet gains in rigidity and tends to shed the deposited absorbent resin powder.
Several methods which comprise directly polymerizing a monomer on a substrate sheet thereby effecting simultaneously the production of an absorbent resin and the formation of a sheet have been proposed (such as are disclosed in JP-A-60-149,609, JP-A-62-53,309, JP-A-62-62,829, and JP-A-62-97,979), for example. These methods invariably suffer from the following drawbacks. The possibility that the polymerization initiator and other additives and a relatively large portion of the monomer used for the polymerization will survive the polymerization is high. Where these residual substances promise dubious safety, they pose a problem when the product of the polymerization is to be used as in sanitary articles or foodstuffs. Further, the reactions involved are difficult to control and deficient in productivity.
Absorbent articles which are intended to be incorporated in such sanitary goods as diapers, pads to be worn by adult incontinent patients, and sanitary articles have been known to the art (such as are disclosed in U.S. Pat. No. 4,699,619, U.S. Pat. No. 4,798,603, and U.S. Pat. No. 4,834,735). Generally, these absorbent articles are formed of a fibrous matrix and further of an absorbent resin as an optional component. The fibrous matrix is formed of either cellulose fibers known as wood pulp fluff or the combination of cellulose fibers with synthetic fibers.
Of the conventional absorbent articles, those which use no absorbent resin necessarily assume a large volume and prove to be unusually inconvenient to handle. Specifically, in these absorbent articles, the amount of water to be absorbed by the wood pulp fluff per unit weight of absorbent article is rather small (about 7 to 9 g/g), indicating that the wood pulp fluff must be used in a relatively large amount for the purpose of increasing the amount of water so absorbed to a desired level and, as a result, the produced absorbent article is relatively large and thick. In contrast to the wood pulp fluff, the absorbent resin has a considerably larger capacity for absorbing water (at least 15 g/g). Thus, the incorporation of an absorbent resin in an absorbent article permits a decrease in the amount of wood pulp fluff to be used. The use of an absorbent resin, therefore, allows production of a small and thin absorbent article. The conventional absorbent articles, however, still have a relatively small absorbent resin content (generally not more than about 50% by weight) and fall short of deserving to be designated as an ample small and thin absorbent article. When the amount of water to be absorbed per unit weight is taken into account, it is logically concluded that the increase in the amount of an absorbent resin ought to allow production of a compact and thin absorbent article. EP-A-443,627 proposes an absorbent article which has an increased absorbent resin content.
When an absorbent article is produced with a well-known absorbent resin incorporated therein at an increased content, various problems may arise. One of the problems consists in the phenomenon of gel blocking. Generally, when an absorbent resin absorbs water and swells, it deforms and fills up the empty spaces which formerly existed between adjacent absorbent resin particles and between the absorbent resin particles and a fibrous matrix and, as a result, obstructs the flow of a fluid formerly existed in the empty spaces. When the amount of an absorbent resin is small, the fibrous matrix is capable of repressing the mutual approach of adjacent absorbent resin particles and retaining a capillary structure enough for flow of the fluid in the matrix. Another problem resides in the insufficiency of the speed at which the absorbent resin absorbs. Generally, the speed of absorption by an absorbent resin is too small as compared with that by wood pulp fluff to cope favorably with the speed of fluid which is adopted in an absorbent article in the course of actual use of the absorbent article. Thus, the fibrous matrix such as of wood pulp fluff is used to function as a temporary storage layer for the fluid being handled by an absorbent article. Such is the true state of affairs.
EP-A-443,627 proposes an absorbent article which has an increased absorbent resin content owing to the use of an absorbent resin adapted to absorb water at a heightened speed. When the fibrous matrix fulfilling the role of fixing a particulate absorbent resin as proposed in EP-A443,627 is present only in a small amount or totally absent, the problem of migration or maldistribution of an absorbent resin within an absorbent article is predictable. Generally, the absorbent resin is in the form of fine powder or of minute particles. Before or during the use of the absorbent article, therefore, the absorbent resin readily migrates within the absorbent article and induces the phenomenon of maldistribution unexpectedly and imparts a stable quality to the absorbent article only with difficulty. In the circumstances, the desirability of developing a method which is capable of disposing (fixing) an absorbent resin at a desired position in an absorbent article in spite of the problematic increase in the amount of the absorbent resin has found growing recognition. Heretofore, the practice of effecting the fixation of a particulate absorbent resin as by sandwiching the absorbent resin between opposed sheets of paper or by mixing the absorbent resin with a fibrous matrix such as of pulp and subsequently molding the resultant mixture in the form of sheet by such a treatment of fixation as the embossing work as already mentioned has been in vogue. These methods, however, encounter difficulties in coping with the case involving the problematic increase in the amount of an absorbent resin.
An attempt to produce a compact and thin absorbent article by increasing the amount of an absorbent resin results in the fact that in the produced absorbent article, the fibrous matrix which has played the role of fixing the absorbent resin is present in a small amount or totally absent. In this case, it is difficult to fix the resin at a prescribed position until after the absorbent resin has fulfilled the role of absorbing water and swelling with the absorbed water. Even when this fixation has been attained somehow or other until immediately before the absorbent article is put to use, it possibly becomes difficult to prevent the migration of gel at the time that the absorbent resin is swelled and gelled by absorbing water.
In the method which effects the treatment of fixation after having imparted an absorbent resin to a substrate, the produced composite is deficient in strength and incapable of preventing gel migration. When the absorbent resin has been thoroughly fixed, the produced composite exhibits an insufficient capacity for absorption. In the case of methods involving distribution of an absorbent resin among a plurality of cells (such as are disclosed in U.S. Pat. No. 5,035,805 and GB 2,251,206), the cells must be given a decreased size for the purpose of repressing the possible migration of the resin in an absorbent article prior to the use of the absorbent article. This decrease of size results in a decrease in the capacity of the absorbent article for absorption. Moreover, the process of forming numerous cells does not always prove to be highly productive.
This invention, therefore, has an object of providing a sheetlike absorbent material supple, tough, pleasant of touch, and excellent in the ability to absorb water and a method for the production thereof.
Another object of this invention is to provide an absorbent material which abounds with suppleness, possesses an ideal ability to absorb water (speed of absorption and amount of water absorbed), succumbs to section into pieces of desired size and shape, and enjoys high safety. A further object of the present invention is to provide a method for the production of the absorbent material mentioned above, which method operates with simplicity, abounds with productivity, and allows easy control of the performance of product. Still another object of this invention is to provide a compact and thin article using the absorbent material mentioned above.
Yet another object of this invention is to provide a sheetlike absorbent material which abounds with suppleness, exhibits an ideal ability to absorb water, and possesses physical properties excellent in stability to endure the effect of aging in an atmosphere of low humidity and a method for the production thereof.
A further object of this invention is to provide a compact and thin absorbent article using the sheetlike absorbent material mentioned above.
Still another object of this invention is to provide a thin absorbent article which possesses a large capacity for absorption of water, permits effective prevention of gel migration within the absorbent article in the course of use, and precludes deformation.